1. Field of the Invention
This invention relates to cation exchange membranes and more particularly to improved cylindrical cation exchange membranes for use in electrolysis of an aqueous alkali metal chloride solution in a finger type electrolytic cell.
2. Description of the Prior Art
The term "finger type electrolytic cell" as used herein includes an electrolytic cell of flattened tube-type construction as well as an electrolytic cell of finger type construction as described in J. S. Sconce, CHLORINE--ITS MANUFACTURE, PROPERTIES AND USES, Reinhold Publishing Corp., New York (1962), page 93, as, in more recent times, electrolytic cells of the flattened tube-type construction are also generally referred to as finger type electrolytic cells.
Cylindrically molded cation exchange membranes are suitable for fitting into the finger type electrolytic cells. However, cylindrically molded cation exchange membranes often provide electrolytic products having higher impurities (e.g., alkali metal chloride) than the products obtained from a filter press type electrolytic cell even if electrolysis is conducted by use of membranes with the same performance (i.e., the same properties of the membranes) and the same current density.
One of the reasons for this is that the areas of anode, membrane and cathode are nearly equal to each other in the filter press type electrolytic cell, whereas the area of membrane is larger than that of anode in the finger type electrolytic cell. Since the average current density of the membrane is lower than that of the anode, and low current density areas are present in localized areas of the membrane, and the concentration of alkali metal chloride in the alkali metal hydroxide formed is high.
Another reason is that the stiffness of the membrane is not sufficient, and it is difficult to effect the electrolysis while keeping the membrane in a flat or curved form. The membrane surface facing the effective electrolytic surface of anode can be kept with relative ease in a flat form. On the other hand, it is difficult to keep the membrane surface not facing the effective electrolytic surface of anode in a flat or curved form, and wrinkles are readily formed therein.
The expression "effective electrolytic surface of anode" as used herein refers to the total area of the anode which faces the cathode, the distance in a straight line from which to the cathode is short, and on which electrolysis materially occurs.
The wrinkles tending to form in the membrane facing the effective electrolytic surface of anode can be prevented by pressing the membrane against the cathode by use of an anode capable of being extended or an anode capable of being controlled with respect to the distance between the anode and cathode so that the anode, membrane and cathode are brought in intimate contact with each other.
However, at the membrane surfaces facing in directions in which the anode cannot be extended or controlled with respect to the distance (that is, those areas not facing the effective electrolytic surface of anode), it is usually difficult to hold the anode, membrane and cathode in an intimate contact state and therefore wrinkles are readily formed in these areas of the membrane surface. This leads to an undesirable situation wherein at the surface facing the anode chlorine gas is trapped or it is at least difficult for the chlorine gas to escape therefrom. This chlorine gas is converted to chlorine ions in a cathodic solution. Therefore, where cylindrically molded cation exchange membranes alone are used, the product obtained is sometimes inferior in quality to that wherein the same membrane is applied using a filter press type electrolytic cell.
Also, where the wrinkles are formed in the membrane surface facing the cathode, hydrogen gas is easily trapped, and it is difficult for the hydrogen gas to escape therefrom. Because of this gas gap, the cell voltage is increased.